Field-of-study mismatch occurs when workers educated in a particular field work in another. It is conceptually distinct from qualifications or skills mismatch, although a part of qualifications and skills mismatch results from graduates from a particular field having to downgrade to find work in another field. Some studies have identified labour market dynamics related to field-of-study mismatch, but few (if any) have sought to directly understand the interplay between labour supply factors (the types of skills brought to the workplace) and the labour demand factors (the types of skills demanded by employers) in field-of-study mismatch. Using data from the Programme for International Assessment of Adult Competencies’ Survey of Adult Skills (PIAAC), this paper shows that although students may choose to specialise in a particular field, it is not solely up to them to actually work in that field. In accordance with assignment theories, both the degree of saturation of a particular field in the labour market and the level of generic skills of a particular field predict the occurrence of field-of-study mismatch, highlighting that mismatch is the result of both labour supply- and demand-side factors. The paper then evaluates the costs to individuals – in terms of wages, risk of being out of work and job satisfaction. Findings suggest that the costs of field-of-study mismatch may only be high in terms of individual earnings when it is associated to qualification mismatch. For economies, field-of-study mismatch, when associated with qualifications mismatch, can amount to important costs, meriting the attention of policy makers to better aligning course places to skill needs or by encouraging skill transferability across fields.